In raster scan type radar apparatuses, the size of a video of a radar is basically determined based on a horizontal beam width and a transmitted pulse width. The wider the horizontal beam width, the larger the expansion in an azimuth direction of echo, and the longer the transmitted pulse width, the larger the expansion in a distance direction of echo. Therefore, due to the expansion of the horizontal beam width of a transmitted wave beam and a received wave beam formed by an antenna, even the same target which is enlarged in the azimuth direction and displayed at a position distant from a sweep center on a display, becomes smaller as the target approaches near a ship carrying the radar apparatus (near the center). This tendency becomes more significant as the resolution of a display is increased (a smaller size of each pixel). In a display having such a high resolution, a target near the position of the own ship is displayed as having a considerably small size. When a sea surface reflection removing process is performed, the size of a target is further reduced due to an influence of the process, so that the target size reduction near the center becomes more significant, resulting in a significant reduction in visibility.
As a radar apparatus which solves such a problem, there is an apparatus which, after drawing an image at a pixel where a target is present, following this write operation, accesses again a pixel adjacent thereto in a direction substantially opposite to a sweep moving direction in a rectangular coordinate system, compares data already stored at the adjacent pixel with the current input data, and writes the greater data into the adjacent pixel (see, for example, Patent Documents 1 and 2).
[Patent Document 1] JP No. 2648983 B
[Patent Document 2] JP No. 2003-28950 A
However, in such a conventional radar apparatus, access (drawing) is performed with respect to the same pixel a plurality of times, so that the number of times of access to an image memory during one cycle of sweeping increases. Therefore, a time required to access the image memory increases with an increase in the number of pixels to be enlarged. Here, when display is performed on a high-resolution display, since each pixel in the image memory also inevitably becomes smaller, the number of pixels to be enlarged increases, so that a time required to write detected image data into the image memory increases. On the other hand, in recent years, some radar apparatuses have an antenna having a high rotational speed so as to support high-speed ships, for example. Therefore, when an attempt is made to perform the above-described enlargement/display process in such a high-speed antenna rotation type radar apparatus, it is highly likely that there is not an enough time to write data into the image memory, so that the entire image memory cannot be updated during one cycle of sweeping.
In conventional radar apparatuses, detected image data is enlarged only in the azimuth direction, so that the shape of a target displayed on a display differs from its actual shape, i.e., becomes unnatural.
Also in conventional radar apparatuses, detected image data is enlarged irrespective of the size of a target, so that detected image data which does not require enlargement is also enlarged, and therefore, display resolution is reduced more than necessary.
An object of the present invention is to provide a radar apparatus in which a rate at which detected image data is written into an image memory, is not reduced, irrespective of the enlarged amount of the detected image data, and like apparatuses thereto.
Another object of the present invention is to provide a radar apparatus which can obtain an enlarged image, depending on detected data (detected signal) of a target, by enlarging detected image data in two-dimensional directions, and like apparatuses thereto.
Still another object of the present invention is to provide a radar apparatus which can reliably display a target around a ship carrying the radar (own ship) without enlarging detected image data when it is not required, and like apparatuses thereto.